British Pat. No. 1,000,164 discloses a screwcap with an inner cap and an outer cap made from easily deformable sheet metal so that the outer cap threads can only be produced during closing of the container by rolling on the container threads. A groove or peripheral rib, for maintaining the inner cap in the outer cap before closing the container, is also produced by roll shaping on the collar part of the outer cap. A guarantee strip is provided at the open end of the outer cap collar, and is connected thereto by means of a peripheral, predetermined breaking point. The guarantee strip embraces an annular rib provided on the container and is also formed by cold working, i.e. by rolling the corresponding part of the outer cap onto the outer periphery of the container. The cap is only raised from the container after destroying the guarantee strip at the predetermined breaking point.
When unscrewing the cap from the container opening, it is not simultaneously necessary to apply a torque for destroying this peripheral predetermined breaking point between the guarantee strip and the remainder of the outer cap, and a torque for raising the inner cap from the container opening. The inner cap is held with sufficient play in the groove or by the peripheral rib in the outer cap such that it is only raised from the container opening, counter to the force of the vacuum in the container and the adhesion of its sealing material to the container opening, after the predetermined breaking point has been destroyed and/or the guarantee strip has already been detached from the outer cap.
Such conventional screwcap is relatively expensive due to its material and the production process. Its partial production by rolling on the periphery of the normally glass container can damage the container edge. In addition, the cap material has a sharp edge and can therefore cause injury. It was considered necessary to use sheet metal for producing the screwcap because, on applying the sealing material, e.g., flexible PVC, to the inside of the cap, it is necessary to heat to approximately 200.degree. C., i.e. to a temperature which would damage normal plastic material.
Maintaining the inner cap in the outer cap by a peripheral rib or in a peripheral groove is disadvantageous. On unscrewing the cap, the rib comes into contact with the entire edge of the inner cap so that the inner cap is simultaneously raised over its entire periphery. Due to the vacuum normally present in the container and the adhesion of the sealing layer of the inner cap to the face of the container opening, the opening forces can be so great, particularly in large diameter containers for baby food and the like, that the easily deformable outer cap is unthreaded over the inner cap without raising it. The inward sloping of the edge of the inner cap in the embodiments disclosed in British Pat. No. 1,000,164 for maintaining the sealing material in place increases this problem. When this occurs, the inner cap must be detached with a tool, e.g. a knife, which must be forced between the opening edge and the inner cap. The aforementioned disadvantages of this conventional cap have prevented it from being used commercially.